The present invention relates to a straight motion assisting device for a work machine, wherein the device is adapted for receiving an input indicative of a steering angle from a position of a moveable member and adapted for applying a restoring force for affecting the work machine towards a straight motion when the steering angle input indicates that the work machine deviates from the straight motion.
The straight motion assisting device is especially adapted for an articulated (or frame-steered) work machine. Such a work machine comprises a front section with a front frame and a first set of ground engaging members, a rear section with a rear frame and a second set of ground engaging members, and at least one pivot joint between the front section and the rear section allowing the front section to pivot with regard to the rear section about an axis extending in a vertical direction of the work machine. Further, the work machine comprises means for steering the work machine via pivoting the front section relative to the rear section about the vertical pivot axis. The steering means is normally formed by a pair of hydraulic cylinders.
The invention will below be described for an articulated hauler. This should however be regarded as a non-limiting example, —wherein the invention may be realized in other types of articulated work machines, such as wheel loaders. Further terms frequently used for work machines are “earth-moving machinery”, “off-road work machines”, “construction equipment” and “forest machines”. The term “ground engaging members” may comprise wheels, caterpillar tracks etc.
In connection with transportation of heavy loads, e.g. in contracting work, a work machine of the type of an articulated hauler is frequently used. Such work machines may be operated with large and heavy loads in areas where there are no roads, for example for transports in connection with road or tunnel building, sand pits, mines, forestry and similar environments. Thus, an articulated hauler is frequently used in rough terrain with various slippery ground.
It is emphasized that a frame-steered work machine is adapted for an optimized off road performance with regard to any vehicle with Ackerman steering (front wheel steering). More specifically, the articulated steering creates conditions for using much larger (both with regard to diameter and width) front wheels than with so-called Ackermann steering.
The geometry of a conventional articulated work machine can make the steering unstable. If the machine is steered out from centre (i.e. a straight forward direction) the driver normally needs to apply a force on the steering wheel in order to get the machine back into straight forward direction.
JP 54-90726 discloses a frame-steered work machine provided with a straight motion assisting device. A motor is directly connected to a steering wheel shaft in order to apply a restoring force on the shaft. A servoamplifier is adapted to control rotation of the motor. A steering angle and a vehicle speed are detected. The motor is controlled to apply the restoring force on the steering wheel shaft when the steering angle deviates from a position indicative of a straight work machine motion and the vehicle speed is above a predetermined limit.
The steering is a function that is critical with regard to safety. There are therefore high quality demands on the computer and associated electronics for the steering function.
It is desirable to achieve a straight motion assisting device for a work machine, which is reliable and cost-efficient.
According to an aspect of the present invention, a work machine includes a straight motion assisting device defined in the introduction above, characterized in that the device comprises a hydraulic system adapted to be effected by the displacement of the moveable member and adapted to apply the restoring force.
One advantage of using a hydraulic system is that the straight motion function can be achieved by means of well known and cost-efficient components.
Thus, the work machine is urged towards a state representing a straight forward (or rearward) direction during the restoring procedure when the steering angle input indicates that the work machine is steered out of said straight forward direction. This is advantageously achieved by controlling the same actuators (normally hydraulic cylinders), which are arranged for performing the steering function.
The position of the moveable member is preferably directly mechanically associated to a steering position of the work machine.
According to a preferred embodiment, the hydraulic system is adapted to apply the restoring force on a member adapted to transmit a motion in a steering apparatus for steering the work machine. This embodiment creates further conditions for a reliable solution with a long life. More specifically, the restoring force is applied on a mechanical link adapted to transmit a steering motion from a steering wheel to the hydraulic steering cylinders. Thus, the steering apparatus is adapted for a mechanical-hydraulic steering.
According to a further development of the embodiment above, the motion transmitting member is formed by the moveable member. Thus, the element that provides the steering angle information is the same as the element that is affected with the restoring force. This embodiment creates conditions for a solution with relatively few parts for achieving the straight motion restoration.
According to a further preferred embodiment, the hydraulic system comprises a hydraulic cylinder with a piston adapted for a reciprocating motion in the cylinder, and that the piston is connected to the moveable member for movement in the cylinder in response to displacement of the moveable member. This embodiment creates conditions for a solution with well known components for achieving the straight motion restoration.
According to a further development of the last mentioned embodiment, the piston defines a first cylinder interior space on a first side of the piston and a second cylinder interior space on a second side of the piston, that the hydraulic system comprises a first hydraulic circuit adapted for flow communication via the first cylinder space and a second hydraulic circuit adapted for flow communication via the second cylinder space when the piston is in a neutral position and that the piston is adapted to close one of said circuits when it is moved a predefined distance from the neutral position. In this way, the interior space which is associated to the closed circuit may be pressurized for pushing the piston back to the neutral position for achieving the straight motion restoration.
According to a further development of the last mentioned embodiment, the hydraulic system comprises a first pump and a second pump, that the first pump is operatively connected to the hydraulic cylinder via said first circuit and the second pump is operatively connected to the hydraulic cylinder via said second-circuit, wherein each pump is adapted to pressurize its associated cylinder interior space when its associated circuit is closed.
According to a further development of the last mentioned embodiment, the pump is rotationally connected to a rotational member, which is adapted to rotate in response to work machine motion. In this way, the pumps are only operated (rotated) when the work machine is in motion. Thus, no restoring force will be applied when the work machine is standing still.
Further preferred embodiment and advantages thereof emerge from the description below, the figures and the claims.